The present invention relates to an ultrasonic imaging apparatus.
An ultrasonic imaging apparatus is utilized as an ultrasonic diagnosis apparatus in the medical field The ultrasonic diagnosis apparatus uses a phased array to electronically focus an ultrasonic beam, thereby outputting an ultrasonic tomographic image with high resolution. A normal ultrasonic diagnosis apparatus can set only one focus point inside an object to be examined. For this reason, the apparatus cannot obtain a high-resolution image in a region outside the focus point. Some methods have been proposed in order to solve this problem. In one method, a focus point is moved in a depth direction during reception. In this method, a circuit which processes echo signals of ultrasonic transducer elements to align the focus point during reception with the wavefronts of echoes incident on the transducer elements is provided. As this circuit, an analog delay circuit using LC delay lines and a CCD (Charge Coupling Device) is adopted, and terminals of the delay lines are switched upon movement of the focus point. This switching operation induces noise, and the noise is mixed into the image signal.
A device for preventing the influence of the switching noise was therefore developed. This device is disclosed in U.S. Pat. No. 4,392,379. With this device, 2-channel reception delay circuits are arranged, so that while the delay time of one delay circuit is switched to change the focus point, a reception signal corresponding to the preceding focus point is fed through the other delay circuit to an output circuit. After the completion of signal processing with respect to one focus point, the delay circuit to be connected to the output circuit is switched, and the same operation is performed. In this way, when the delaycircuits are alternately connected to the output circuit, the focus point is sequentially moved. With this device, since no signal passes through the delay circuit whose taps are changed, no noise is generated during tap changing. However, since the 2-channel delay circuits are required, this results in a bulky and expensive apparatus. Furthermore, when the delay circuits are alternately connected to the output circuit, some switching noise is inevitably generated, and noise cannot be completely eliminated.